Lubricating oil composition

ABSTRACT

Disclosed is a lubricating oil composition comprising: a lubricating base oil having a kinematic viscosity at 100° C. of 3.5 to 4.5 mm 2 /s, a viscosity index of 145 or greater, and an urea adduct value of 2 to 7% by mass; and a poly(meth)acrylate pour-point depressant containing structural units represented by the formula (1): 
     
       
         
         
             
             
         
       
     
     wherein R 1  represents a hydrogen atom and the like, R 2  represents an alkyl group; based on the total amount of the units contained in the depressant, a ratio of the units wherein the R 2  is a methyl group is 0 to 10 mol %, a ratio of the units wherein the R 2  is an alkyl group having 12C or greater is 90 to 100 mol %; the alkyl group having 12C or greater has an average number of 13-16C; the depressant has a weight-average molecular weight of 10,000 to 200,000.

TECHNICAL FIELD

The present invention relates to a lubricating oil composition.

BACKGROUND ART

In terms of energy saving such as fuel efficiency in internal combustion engine lubricating oils, attempts have conventionally been made to satisfy both high viscosity index and low temperature viscosity characteristics of lubricating oil compositions.

As the method for increasing the viscosity index of lubricating oil compositions, there is a method using lubricating base oils having a high viscosity index, such as highly purified mineral oils. As the method for producing high-viscosity-index base oils, a method for refining a lubricating base oil by hydrocracking/hydroisomerization of raw oil containing natural or synthetic normal paraffins is known (see, for example, Patent Literatures 1 to 3).

Moreover, as the method for improving the low temperature viscosity characteristics of lubricating oil compositions, there is a method in which a pour-point depressant is added to lubricating oil compositions (see, for example, Patent Literatures 4 to 6).

CITATION LIST Patent Literature

-   Patent Literature 1: JP-A-2005-154760 -   Patent Literature 2: JP-T-2006-502298 -   Patent Literature 3: JP-T-2002-503754 -   Patent Literature 4: JP-A-H04-036391 -   Patent Literature 5: JP-A-H04-068082 -   Patent Literature 6: JP-A-H04-120193

SUMMARY OF INVENTION Technical Problem

However, the recent demand for energy saving has been increasing. Then, the examination by the present inventor revealed that it was not always easy to sufficiently increase both high viscosity index and cold flow property of lubricating oil compositions even by using a combination of the above-mentioned conventional high-viscosity-index base oil and a pour-point depressant.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a lubricating oil composition having sufficiently increased high viscosity index and cold flow property.

Solution to Problem

In order to solve the above problem, the present inventor first examined the cause of the difference in combined effects depending on the combination of high-viscosity-index base oils and pour-point depressants. The results revealed that in the case of lubricating base oils highly refined by hydrocracking/hydroisomerization, even when the degree of isomerization (i.e., isoparaffin/normal paraffin ratio) was almost the same, the effects of pour-point depressants were different depending on the difference in the molecular structure of isoparaffins (e.g., a large or small number of carbon atoms between the terminal to the branch position). In particular, it was revealed that the effect of adding a pour-point depressant was likely to be insufficient in the case of lubricating base oils with a relatively high content of isoparaffins having a larger number of carbon atoms between the terminal and the branch position.

Then, as a result of further studies based on the above findings, the present inventor found that even when using a lubricating base oil with a high content of isoparaffins having a larger number of carbon atoms between the terminal and the branch position, the high viscosity index and cold flow property could be sufficiently increased by adding a specific poly(meth)acrylate pour-point depressant to the lubricating base oil. Thus, the present invention has been completed.

That is, the present invention provides a lubricating oil composition comprising:

a lubricating base oil having a kinematic viscosity at 100° C. of 3.5 to 4.5 mm²/s, a viscosity index of 145 or greater, and an urea adduct value of 2 to 7% by mass; and

a poly(meth)acrylate pour-point depressant containing structural units represented by the following formula (1):

wherein in formula (1), R¹ represents a hydrogen atom or a methyl group, and R² represents an alkyl group;

based on the total amount of structural units represented by formula (1) contained in the poly(meth)acrylate pour-point depressant, the ratio of structural units represented by formula (1) wherein R² is a methyl group is 0 to 10 mol %, and the ratio of structural units represented by formula (1) wherein R² is an alkyl group having 12 or greater carbon atoms is 90 to 100 mol %;

the alkyl groups having 12 or greater carbon atoms in the poly(meth)acrylate pour-point depressant have an average number of carbon atoms of 13 to 16; and

the poly(meth)acrylate pour-point depressant has a weight-average molecular weight of 10,000 to 200,000.

The kinematic viscosity at 100° C., the viscosity index, and the below-mentioned kinematic viscosity at 40° C. as mentioned in the present invention are the kinematic viscosity at 100° C., the viscosity index, and the kinematic viscosity at 40° C., respectively, measured according to JIS K 2283-1993.

Moreover, the urea adduct value as mentioned in the present invention is measured in the following manner. After 100 g of weighed sample oil (lubricating base oil) is placed in a round-bottom flask, 200 g of urea, 360 ml of toluene, and 40 ml of methanol are added thereto, and the mixture is stirred at room temperature for 6 hours. As a result, white granular crystals are generated as an urea adduct in the reaction solution. The reaction solution is filtered through a 1-micron filter to thereby collect the generated white granular crystals. The obtained crystals are washed 6 times with 50 ml of toluene. The collected white crystals are placed in a flask, 300 ml of pure water and 300 ml of toluene are added, and the mixture is stirred at 80° C. for 1 hour. The aqueous phase is separated and removed with a separatory funnel, and the toluene phase is washed 3 times with 300 ml of pure water. After a desiccant (sodium sulfate) is added to the toluene phase, followed by dehydrating treatment, the toluene is removed. The ratio (percentage by mass) of the thus-obtained urea adduct to the sample oil is defined as the urea adduct value.

Furthermore, the weight-average molecular weight as mentioned in the present invention is weight-average molecular weight (styrene conversion) determined by gel permeation chromatographic analysis (GPC).

Advantageous Effects of Invention

The present invention provides a lubricating oil composition having sufficiently increased high viscosity index and cold flow property.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention are described in detail below.

The lubricating oil composition according to the embodiment of the present invention comprises a lubricating base oil having a kinematic viscosity at 100° C. of 3.5 to 4.5 mm²/s, a viscosity index of 145 or greater, and a urea adduct value of 2 to 7% by mass; and a poly(meth)acrylate pour-point depressant containing structural units represented by the following formula (1):

wherein in formula (1), R¹ represents a hydrogen atom or a methyl group, and R² represents an alkyl group. Based on the total amount of structural units represented by formula (1) contained in the poly(meth)acrylate pour-point depressant, the ratio of structural units represented by formula (1) wherein R² is a methyl group is 0 to 10 mol %, and the ratio of structural units represented by formula (1) wherein R² is an alkyl group having 12 or greater carbon atoms is 90 to 100 mol %. The average number of carbon atoms in the alkyl groups having 12 or greater carbon atoms in the poly(meth)acrylate pour-point depressant is 13 to 16. Moreover, the weight-average molecular weight of the poly(meth)acrylate pour-point depressant is 10,000 to 200,000.

In the present embodiment, the kinematic viscosity at 100° C. of the lubricating base oil is 3.5 to 4.5 mm²/s, and preferably 3.6 to 4.3 mm²/s. Moreover, the viscosity index of the lubricating base oil is 145 or greater, preferably 147 or greater, and more preferably 150 or greater, in terms of viscosity-temperature characteristics. Although the kinematic viscosity at 40° C. of the lubricating base oil is not particularly limited, it is preferably 14 to 20 mm²/s, and more preferably 15 to 19 mm²/s.

Moreover, the urea adduct value of the lubricating base oil is 2 to 7% by mass, as described above, preferably 3 to 7% by mass, and more preferably 4 to 7% by mass, in terms of improving low temperature viscosity characteristics without impairing viscosity-temperature characteristics. Further, an urea adduct value within the above range is preferable, because when dewaxing treatment is performed in the production process of the lubricating base oil, dewaxing conditions can be relieved, and because economic efficiency is excellent.

In the present embodiment, the method for producing the lubricating base oil is not particularly limited, as long as the kinematic viscosity at 100° C., a viscosity index, and an urea adduct value satisfy the above conditions. For example, the lubricating base oil according to the present embodiment can be suitably obtained by performing hydrocracking using whole vacuum gas oil (WVGO), mild-hydrocracked (MHC) oil of WVGO (HIX), deasphalted oil (DAO), MHC oil of DAO, a mixture of these oils, or a mixture of two or greater these oils, as a raw material in the presence of a hydrocracking catalyst, and further performing treatment by a combination of dearomatization and dewaxing.

The WVGO is a distillate oil obtained when residual oil from a crude oil atmospheric distillation apparatus is distilled in a vacuum distillation apparatus, and the WVGO preferably has a boiling point of 360° C. to 530° C.

The HIX is a heavy vacuum gas oil generated by MHC treatment (which is relatively mild hydrocracking in which the degradation rate of 360° C.⁺ fraction is in the range of 20 to 30 wt % under the following reaction conditions: the total pressure is 100 kg/cm² or less, and preferably 60 to 90 kg/cm²; the temperature is 370 to 450° C., and preferably 400 to 430° C.; and LHSV is 0.5 to 4.0 hr⁻¹, and preferably 1.0 to 2.0 hr⁻¹) of WVGO. As the catalyst in the MHC treatment, a catalyst in which a Group-VI metal and a Group-VIII metal are supported by a composite oxide support (e.g., alumina, silica alumina, or alumina boria) and sulfurated can be used. A promoter, such as a phosphorus compound, may be added to alumina. The amounts of the supported metals are such that based on the oxide, the amount of Group-VI metal (e.g., molybdenum, tungsten, or chromium) is 5 to 30 wt %, and preferably 10 to 25 wt %; and the amount of Group-VIII metal (e.g., cobalt or nickel) is 1 to 10 wt %, and preferably 2 to 10 wt %. When WVGO and HIX are mixed, it is preferable to mix 50 wt % or greater of HIX with WVGO.

The deasphalted oil is an oil that does not substantially contain asphaltene, obtained in such a manner that residual oil from a crude oil atmospheric distillation apparatus is distilled in a vacuum distillation apparatus, and the obtained residual oil is treated by propane deasphalting method, and the like.

The hydrocracking of the raw oil can be performed in the presence of a hydrocracking catalyst under the following reaction conditions: the total pressure is medium to low pressure, i.e., 150 kg/cm² or less, and preferably 100 to 130 kg/cm²; the temperature is 360 to 440° C., and preferably 370 to 430° C.; LHSV is as low as 0.5 hr⁻¹ or less, and preferably 0.2 to 0.3 hr⁻¹; and the hydrogen/raw oil ratio is 1,000 to 6,000 s.c.f/bbl-raw oil, and preferably 2,500 to 5,000 s.c.f/bbl-raw oil. In the hydrocracking of the raw oil, the reaction conditions are adjusted so that the degradation rate of 360° C. fraction in the raw oil is 40 wt % or greater, preferably 45 wt % or greater, and more preferably 50 wt % or greater. When HIX is used as the raw oil, the total degradation rate by MHC treatment and hydrocracking is 60 wt % or greater, and preferably 70 wt % or greater. Moreover, when part of undegraded oil is recycled, the degradation rate as mentioned herein is the degradation rate per fresh field, rather than the degradation rate including recycled oil.

The catalyst used in the hydrocracking is preferably a bifunctional catalyst. Specifically, for example, a catalyst having a hydrogenation point derived from a Group-VIb metal and a Group-VIII iron-group metal, and a degradation point derived from a composite oxide of Group-III, Group-IV, and Group-V elements is used. Examples of the Group-VIb metal include tungsten and molybdenum, and examples of the Group-VIII iron-group metal include nickel, cobalt, and iron. These metals are supported by a composite oxide support, and finally used as sulfide.

Examples of the composite oxide used as the support include silica alumina, silica zirconia, silica titania, silica magnesia, silica alumina zirconia, silica alumina titania, silica alumina magnesia, and the like. Crystalline silica alumina (zeolite), crystalline alumina phosphate (ALPO), and crystalline silica alumina phosphate (SAPO) may also be used.

The amounts of the metals supported by the composite oxide are such that based on the oxide, the amount of Group-VIb metal is 5 to 30% by mass, and preferably 10 to 25% by mass; and the amount of Group-VIII iron-group metal is 1 to 20% by mass, and preferably 5 to 15% by mass.

In addition, for sure, when the raw oil is hydrocracked, the upstream side of a hydrocracking catalyst-packed bed may be filled with a pretreatment catalyst having excellent desulfurization and/or denitrification ability. As this type of pretreatment catalyst, a catalyst in which a Group-VI metal and a Group-VIII metal are supported by a support (e.g., alumina or alumina boria) and sulfurated can be used. A promoter, such as a phosphorus compound, may be added to alumina or alumina boria.

After the raw oil is hydrocracked, lubricating oil fractions may be collected from the degradation product by an ordinary distillation operation, if necessary. Examples of lubricating oil fractions that can be collected include 70 pale fraction with a boiling point range of 343° C. to 390° C., SAE-10 fraction with a boiling range of 390° C. to 445° C., SAE-20 fraction with a boiling range of 445° C. to 500° C., SAE-30 fraction with a boiling range of 500° C. to 565° C., and the like.

The above hydrocracked product, from which lubricating oil fractions have been optionally separated and collected, is subjected to dearomatization treatment after dewaxing treatment, or subjected to dewaxing treatment after dearomatization treatment.

As the dewaxing treatment, solvent dewaxing treatment or catalytic dewaxing treatment can be employed.

The solvent dewaxing treatment can be performed by an ordinary method, such as MEK method. The solvent used in the MEK method includes benzene, toluene, acetone, or a mixed solvent of benzene, toluene, methyl ethyl ketone (MEK), and the like. As for the treatment conditions, the cooling temperature is adjusted so that the dewaxed oil has a predetermined pour-point. The volume ratio of solvent to oil is 0.5 to 5.0, and preferably 1.0 to 4.5. The temperature is −5 to −45° C., and preferably −10 to −40° C.

The catalytic dewaxing treatment can be performed by an ordinary method. For example, pentasil zeolite is used as a catalyst, and the reaction temperature is adjusted so that the dewaxed oil has a predetermined pour-point under a hydrogen flow; however, the reaction conditions thereof are generally such that the total pressure is 10 to 70 kg/cm², and preferably 20 to 50 kg/cm²; and the temperature is 240 to 400° C., and preferably 260 to 380° C. LHSV is in the range of 0.1 to 3.0 hr⁻¹, and preferably 0.5 to 2.0 hr⁻¹.

As the dearomatization treatment, solvent dearomatization treatment and high-pressure hydrogenation dearomatization treatment both can be employed; however, solvent dearomatization treatment is preferred.

Although the solvent dearomatization treatment generally uses a solvent, such as furfural or phenol, it is preferable to use furfural as the solvent in the present invention. The solvent dearomatization treatment is performed under the following conditions: the solvent/oil volume ratio is 4 or less, preferably 3 or less, and more preferably 2 or less; the temperature is 90 to 150° C.; and the raffinate yield is 60 volume % or greater, preferably 70 volume % or greater, and more preferably 85 volume % or greater.

The dearomatization treatment by high-pressure hydrogenation reaction is generally performed in the presence of a catalyst in which a Group-VIb metal and a Group-VIII iron-group metal are supported by an alumina support and sulfurated, under the following conditions: the total pressure is 150 to 200 kg/cm², and preferably 70 to 200 kg/cm²; the temperature is 280 to 350° C., and preferably 300 to 330° C.; and LHSV is 0.2 to 2.0 hr⁻¹, and preferably 0.5 to 1.0 hr⁻¹. The amounts of the metals supported in the catalyst are such that based on the oxide, the amount of Group-VIb metal (e.g., molybdenum, tungsten, or chromium) is 5 to 30% by mass, and preferably 10 to 25% by mass; and the amount of Group-VIII iron-group metal (e.g., cobalt or nickel) is 1 to 10% by mass, and preferably 2 to 10% by mass.

When solvent dearomatization treatment is used as the dearomatization treatment, hydrotreatment can be performed, if necessary, after the solvent dearomatization treatment. The hydrotreatment is performed by bringing the solvent-dearomatized oil into contact with a hydrogenation catalyst in which a Group-VIb metal and a Group-VIII iron-group metal are supported by an alumina support and sulfurated, under hydrogenation reaction conditions where the total reaction pressure is as low as 50 kg/cm² or less, and preferably 25 to 40 kg/cm². The hydrotreatment under such a relatively low pressure remarkably improves the light stability of the solvent-dearomatized oil. The amounts of the supported metals are such that based on the oxide, the amount of Group-VIb metal (e.g., molybdenum, tungsten, or chromium) is 5 to 30% by mass, and preferably 10 to 25% by mass; and the amount of Group-VIII iron-group metal (e.g., cobalt or nickel) is 1 to 10% by mass, and preferably 2 to 10% by mass.

In the method for producing the lubricating base oil according to the present embodiment, when lubricating oil fractions are not collected from the hydrocracked product of the raw oil in the production process, the lubricating oil fractions can be collected by an ordinary distillation operation after dearomatization treatment, dewaxing treatment, or hydrotreatment.

Other properties of the lubricating base oil according to the present embodiment are not particularly limited, as long as the kinematic viscosity at 100° C., viscosity index, and urea adduct value satisfy the above conditions; however, it is preferable that the lubricating base oil according to the present embodiment satisfies the following conditions.

The content of saturated components in the lubricating base oil according to the present embodiment is preferably 90% by mass or greater, more preferably 93% by mass or greater, and even more preferably 95% by mass or greater, based on the total amount of the lubricating base oil. Moreover, the ratio of cyclic saturated components in the saturated components is preferably 0.1 to 60% by mass, more preferably 0.5 to 55% by mass, even more preferably 1 to 52% by mass, and particularly preferably 5 to 50% by mass. When the content of saturated components and the ratio of cyclic saturated components in the saturated components satisfy the above conditions, viscosity-temperature characteristics and thermal oxidation stability can be achieved. Further, when an additive is added to the lubricating base oil, the function of the additive can be exhibited at a higher level while the additive is sufficiently stably dissolved and held in the lubricating base oil. Furthermore, when the content of saturated components and the ratio of cyclic saturated components in the saturated components satisfy the above conditions, the friction characteristics of the lubricating base oil itself can be improved. As a result, an improvement in friction reduction effect and an improvement in energy saving can be achieved.

When the content of saturated components is less than 90% by mass, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics tend to be insufficient. Moreover, when the ratio of cyclic saturated components in the saturated components is less than 0.1% by mass, the solubility of an additive, when added to the lubricating base oil, is insufficient, and the effective amount of the additive dissolved and held in the lubricating base oil is reduced. Thus, there is a tendency that the function of the additive cannot effectively be obtained. Furthermore, when the ratio of cyclic saturated components in the saturated components is more than 60% by mass, the effect of an additive, when added to the lubricating base oil, tends to be reduced.

In the present embodiment, that the ratio of cyclic saturated components in the saturated components is 30 to 50% by mass is equivalent to that the ratio of non-cyclic saturated components in the saturated components is 70 to 50% by mass. Here, the non-cyclic saturated components include both normal paraffins and isoparaffins. The ratios of normal paraffins and isoparaffins in the lubricating base oil according to the present embodiment are not particularly limited, as long as the urea adduct value satisfies the above conditions; however, the ratio of isoparaffins is preferably 40 to 70% by mass, more preferably 42 to 65% by mass, even more preferably 44 to 60% by mass, and particularly preferably 45 to 55% by mass, based on the total amount of the lubricating base oil. When the ratio of isoparaffins in the lubricating base oil satisfies the above conditions, viscosity-temperature characteristics and thermal oxidation stability can be further improved. Further, when an additive is added to the lubricating base oil, the function of the additive can be exhibited at a further higher level while the additive is sufficiently stably dissolved and held.

The content of saturated components as mentioned in the present invention is a value (unit: % by mass) measured according to ASTM D 2007-93.

Moreover, the ratios of cyclic saturated components and non-cyclic saturated components in the saturated components as mentioned in the present invention are, respectively, a naphthene content (measuring object: monocyclic to hexacyclic naphthenes; unit: % by mass) and an alkane content (unit: % by mass) measured according to ASTM D 2786-91.

Moreover, the ratio of normal paraffins in the lubricating base oil as mentioned in the present invention is a value obtained in such a manner that the saturated components separated and fractionated by the method described in ASTM D2007-93 are subjected to gas chromatographic analysis under the following conditions, and the measured value obtained by identifying and quantifying the ratio of normal paraffins in the saturated components is converted based on the total amount of the lubricating base oil. In the identification and quantification, a mixed sample of normal paraffins having 5 to 50 carbon atoms is used as the standard sample. The amount of normal paraffins in the saturated components is determined as the ratio of the sum of the peak areas corresponding to each normal paraffin with respect to the total peak area (except for the peak area derived from the diluent) in the chromatogram.

(Conditions of Gas Chromatography)

Column: Liquid phase non-polar column (length: 25 mm, inner diameter: 0.3 mmφ, and liquid phase film thickness: 0.1 μm) Temperature increase conditions: 50° C. to 400° C. (heating rate: 10° C./min) Carrier gas: helium (linear velocity: 40 cm/min) Split ratio: 90/1 Sample injection amount: 0.5 μL (injection amount of sample diluted 20 times with carbon disulfide)

Moreover, the ratio of isoparaffins in the lubricating base oil is a value obtained by converting the difference between the amount of non-cyclic saturated components in the saturated components and the amount of normal paraffins in the saturated components, based on the total amount of the lubricating base oil.

In the method for separating saturated components, or the composition analysis of cyclic saturated components, non-cyclic saturated components, and the like, similar methods by which the same results are obtained can be used. For example, in addition to the above methods, a method described in ASTM D2425-93, a method described in ASTM D2549-91, a method using high-speed liquid chromatography (HPLC), or modified versions of these methods can be used.

Moreover, the content of aromatic components in the lubricating base oil according to the present embodiment is preferably 5% by mass or less, more preferably 0.05 to 3% by mass, even more preferably 0.1 to 1% by mass, and particularly preferably 0.1 to 0.5% by mass, based on the total amount of the lubricating base oil. When the content of aromatic components is more than the above upper limit, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics, as well as volatilization inhibiting properties and low temperature viscosity characteristics tend to decrease. Further, when an additive is added to the lubricating base oil, the effect of the additive tends to be reduced. In addition, the lubricating base oil according to the present embodiment may not contain aromatic components; however, the solubility of the additive can be further increased by setting the content of aromatic components to be 0.05% by mass or greater.

The content of aromatic components as mentioned herein is a value measured according to ASTM D2007-93. The aromatic components generally include alkylbenzene and alkylnaphthalene, as well as anthracene, phenanthrene, and alkylated products thereof, and further include compounds in which four or greater benzene rings are condensed, and aromatic compounds having heteroatoms, such as pyridines, quinolines, phenols, and naphthols.

Moreover, the % C_(P) of the lubricating base oil according to the present embodiment is preferably 80 or greater, more preferably 82 to 99, even more preferably 85 to 98, and particularly preferably 90 to 97. When the % C_(P) of the lubricating base oil is less than 80, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics tend to decrease. Further, when an additive is added to the lubricating base oil, the effect of the additive tends to be reduced. In contrast, when the % C_(P) of the lubricating base oil is more than 99, the solubility of the additive tends to decrease.

Moreover, the % C_(N) of the lubricating base oil according to the present embodiment is preferably 20 or less, more preferably 15 or less, even more preferably 1 to 12, and still more preferably 3 to 10. When the % C_(N) of the lubricating base oil is more than 20, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics tend to decrease. In contrast, when the % C_(N) is less than 1, the solubility of the additive tends to decrease.

Moreover, the % C_(A) of the lubricating base oil according to the present embodiment is preferably 0.7 or less, more preferably 0.6 or less, and even more preferably 0.1 to 0.5. When the % C_(A) of the lubricating base oil is more than 0.7, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics tend to decrease. The % C_(A) of the lubricating base oil according to the present embodiment may be 0; however, the solubility of the additive can be further increased by setting the % C_(A) to be 0.1 or greater.

Furthermore, the ratio of % C_(P) to % C_(N) (% C_(P)/% C_(N)) in the lubricating base oil according to the present embodiment is preferably 7 or greater, more preferably 7.5 or greater, and even more preferably 8 or greater. When the % C_(P)/% C_(N) is less than 7, viscosity-temperature characteristics, thermal oxidation stability, and friction characteristics tend to decrease. Further, when an additive is added to the lubricating base oil, the effect of the additive tends to be reduced. Moreover, the % C_(P)/% C_(N) is preferably 200 or less, more preferably 100 or less, even more preferably 50 or less, and particularly preferably 25 or less. The solubility of the additive can be further increased by setting the % C_(P)/% C_(N) to be 200 or less.

The % C_(P), % C_(N), and % C_(A) as mentioned in the present invention are the percentage of the number of paraffin carbon atoms to the total number of carbon atoms, the percentage of the number of naphthene carbon atoms to the total number of carbon atoms, and the percentage of the number of aromatic carbon atoms to the total number of carbon atoms, respectively, all of which are determined by the method according to ASTM D3238-85 (n-d-M ring analysis). That is, the above-mentioned preferred ranges of the % C_(P), % C_(N), and % C_(A) are based on values determined by the above method. For example, the % C_(N) of a lubricating base oil that does not contain naphthene components determined by the above method may be a value exceeding 0.

Moreover, the iodine value of the lubricating base oil according to the present embodiment is preferably 0.5 or less, more preferably 0.3 or less, and even more preferably 0.15 or less. The iodine value may also be less than 0.01; however, because the effect matching the value is low, and in terms of the relationship with economic efficiency, the iodine value is preferably 0.001 or greater, and more preferably 0.05 or greater. Thermal oxidation stability can be remarkably improved by setting the iodine value of the lubricating base oil to be 0.5 or less. The iodine value as mentioned in the present invention is an iodine value measured by the indicator titration method for “acid value, saponification value, iodine value, hydroxyl value and unsaponifiable matter of chemical products” according to JIS K 0070.

Moreover, the sulfur content of the lubricating base oil according to the present embodiment depends on the sulfur content of the raw material. In the lubricating base oil according to the present embodiment, the sulfur content is preferably 10 mass ppm or less, more preferably 5 mass ppm or less, and even more preferably 3 mass ppm or less, in terms of further improvement in thermal oxidation stability and a low sulfurization.

Moreover, the nitrogen content of the lubricating base oil according to the present embodiment is not particularly limited, but is preferably 5 mass ppm or less, more preferably 3 mass ppm or less, and even more preferably 1 mass ppm or less. When the nitrogen content is more than 5 mass ppm, thermal oxidation stability tends to decrease. The nitrogen content as mentioned in the present invention is a nitrogen content measured according to JIS K 2609-1990.

Moreover, the pour-point of the lubricating base oil according to the present embodiment is preferably −7.5° C. or less, more preferably −10° C. or less, and even more preferably −12.5° C. or less. When the pour-point is more than the above upper limit, the cold flow property of the entire lubricating oil using the lubricating base oil tends to decrease. The pour-point as mentioned in the present invention is a pour-point measured according to JIS K 2269-1987.

Moreover, the CCS viscosity at −35° C. of the lubricating base oil according to the present embodiment is preferably 2,000 mPa·s or less, more preferably 1,800 mPa·s or less, and even more preferably 1,700 mPa·s or less. When the CCS viscosity at −35° C. is more than the above upper limit, the cold flow property of the entire lubricating oil using the lubricating base oil tends to decrease. The CCS viscosity at −35° C. as mentioned in the present invention is a viscosity measured according to JIS K 2010-1993.

Moreover, the density at 15° C. (ρ₁₅, unit: g/cm³) of the lubricating base oil according to the present embodiment is preferably equal to or less than the value of p represented by the following formula (2), that is, ρ₁₅≦ρ:

ρ=0.0025×kv100+0.816  (2)

wherein kv100 represents the kinematic viscosity at 100° C. (mm²/s) of

the lubricating base oil.

When ρ₁₅>ρ holds, viscosity-temperature characteristics and thermal oxidation stability, as well as volatilization inhibiting properties and low temperature viscosity characteristics tend to decrease. Further, when an additive is added to the lubricating base oil, the effect of the additive tends to be reduced.

More specifically, the ρ₁₅ of the lubricating base oil according to the present embodiment is preferably 0.85 g/cm³ or less, and more preferably 0.84 g/cm³ or less.

The density at 15° C. as mentioned in the present invention is a density measured at 15° C. according to JIS K 2249-1995.

Moreover, the aniline point (AP (° C.)) of the lubricating base oil according to the present embodiment depends on the viscosity grade of the lubricating base oil, but is preferably higher than the value of A represented by the following formula (3), that is, AP≧A:

A=4.3×kv100+100  (3)

wherein kv100 represents the kinematic viscosity at 100° C. (mm²/s) of the lubricating base oil.

When AP<A holds, viscosity-temperature characteristics and thermal oxidation stability, as well as volatilization inhibiting properties and low temperature viscosity characteristics tend to decrease. Further, when an additive is added to the lubricating base oil, the effect of the additive tends to be reduced.

More specifically, the AP of the lubricating base oil according to the present embodiment is preferably 110° C. or greater, and more preferably 115° C. or greater.

The aniline point as mentioned in the present invention is an aniline point measured according to JIS K 2256-1985.

Moreover, the distillation characteristics of the lubricating base oil according to the present embodiment are preferably such that in gas chromatography distillation, the initial boiling point (IBP) is 350 to 370° C., and the final boiling point (FBP) is 480 to 520° C. A lubricating base oil having the above-mentioned preferred viscosity range can be obtained by refining one or greater fractions selected from fractions within this distillation range.

The IBP and FBP as mentioned in the present invention are distillation points measured according to ASTM D2887-97.

Moreover, the residual metal content of the lubricating base oil according to the present embodiment is derived from metal components contained in the catalyst and raw material unavoidably mixed during the production process; however, it is preferable to sufficiently remove the residual metal components. For example, the contents of Al, Mo, and Ni are each preferably 1 mass ppm or less. When the contents of these metal components are more than the above upper limit, the function of the additive added to the lubricating base oil tends to be inhibited.

The residual metal content as mentioned in the present invention is a metal content measured according to JPI-5S-38-2003.

Next, the poly(meth)acrylate-based viscosity index improver according to the present embodiment is described.

The poly(meth)acrylate-based viscosity index improver according to the present embodiment contains structural units represented by the above formula (1). The ratio of structural units represented by formula (1) wherein R² is a methyl group is 0 to 10 mol %, preferably 0 to 5 mol %, and particularly preferably 0 mol %, based on the total amount of structural units represented by formula (1) contained in the poly(meth)acrylate pour-point depressant. When this ratio is more than the above upper limit, the low temperature viscosity characteristics of the lubricating oil composition are insufficient.

Moreover, the ratio of structural units represented by formula (1) wherein R² is an alkyl group having 12 or greater carbon atoms is 90 to 100 mol %, preferably 95 to 100 mol %, and particularly preferably 100 mol %, based on the total amount of structural units represented by formula (1) contained in the poly(meth)acrylate pour-point depressant. When this ratio is less than the above lower limit, the low temperature viscosity characteristics of the lubricating oil composition are insufficient. The alkyl group having 12 or greater carbon atoms may be either a linear alkyl group or a branched alkyl group.

Moreover, among of R²s in the poly(meth)acrylate pour-point depressant, the alkyl groups having 12 or greater carbon atoms have an average number of carbon atoms of 13 to 16, and preferably 13.5 to 15.5. When the average number of carbon atoms is less than the above lower limit or greater than the above upper limit, the low temperature viscosity characteristics of the lubricating oil composition are insufficient. The average number of carbon atoms as mentioned herein is the average number of carbon atoms calculated based on the raw material methacrylate used in the synthesis of the poly(meth)acrylate pour-point depressant.

Moreover, the weight-average molecular weight of the poly(meth)acrylate pour-point depressant is 10,000 to 200,000, preferably 30,000 to 180,000, and more preferably 40,000 to 170,000. When the weight-average molecular weight is less than the above lower limit or greater than the above upper limit, the low temperature viscosity characteristics of the lubricating oil composition are insufficient.

Furthermore, the ratio of structural units represented by formula (1) wherein R² is an alkyl group having 20 or greater carbon atoms is preferably 0 to 10 mol %, more preferably 0 to 5 mol %, and particularly preferably 0 mol %, based on the total amount of structural units represented by formula (1) contained in the poly(meth)acrylate pour-point depressant. When this ratio is more than the above upper limit, the low temperature viscosity characteristics of the lubricating oil composition are insufficient.

The poly(meth)acrylate-based viscosity index improver according to the present embodiment can be obtained by selecting, among (meth)acrylates represented by the following formula (4), one wherein R² satisfies the above conditions to prepare a raw material monomer, and polymerizing the raw material monomer so that the weight-average molecular weight satisfies the above conditions. The raw material monomer may further include monomers other than the (meth)acrylate represented by the following formula (4). In that case, the content ratio of the (meth)acrylate represented by formula (4) is preferably 70 mol % or greater, and more preferably 80 mol % or greater, based on the total amount of the raw material monomers. [Chemical Formula 3]

wherein in the above formula (4), R¹ represents a hydrogen atom or a methyl group, and R² represents an alkyl group.

The content ratio of the poly(meth)acrylate pour-point depressant according to the present embodiment is preferably 0.01 to 2.0% by mass, more preferably 0.01 to 1.5% by mass, even more preferably 0.01 to 1.0% by mass, and particularly preferably 0.01 to 0.75% by mass, based on the total amount of the lubricating oil composition. When the content of the poly(meth)acrylate pour-point depressant is less than the above lower limit, the low temperature viscosity characteristics of the lubricating oil composition tend to be insufficient. In contrast, when the content of the poly(meth)acrylate pour-point depressant is more than the above upper limit, there is a tendency that kinematic viscosity increases, viscosity temperature characteristics decrease, and energy-saving characteristics decrease.

In order to further improve its performance, the lubricating oil composition according to the present embodiment can contain any additives that are generally used in lubricating oils depending on the purpose. Examples of such additives include pour-point depressants other than the poly(meth)acrylate-based pour-point depressant according to the present embodiment, metal-based detergents, ashless dispersants, anti-wear agents (extreme-pressure agents, oily agents, and the like), friction modifiers, viscosity index improvers, antioxidants, corrosion inhibitors, anti-rust agents, demulsifiers, metal deactivators, antifoaming agents, and other additives.

When the lubricating oil composition according to the present embodiment contains these additives, the content ratio of each additive is preferably 0.01 to 10% by mass based on the total amount of the lubricating oil composition.

The kinematic viscosity at 100° C. of the lubricating oil composition according to the present embodiment is 7.0 to 9.0 mm²/s, more preferably 7.2 to 8.8 mm²/s, even more preferably 7.3 to 8.6 mm²/s, and particularly preferably 7.3 to 8.5 mm²/s. When the kinematic viscosity at 100° C. is less than the above lower limit, lubricity may be insufficient. In contrast, when the kinematic viscosity at 100° C. is more than the above upper limit, required low temperature viscosity and sufficient fuel-efficient performance may not be obtained.

Moreover, the kinematic viscosity at 40° C. of the lubricating oil composition according the present embodiment is preferably 28 to 40 mm²/s, more preferably 30 to 38 mm²/s, even more preferably 31 to 36 mm²/s, and particularly preferably 32 to 35 mm²/s. When the kinematic viscosity at 40° C. is less than the above lower limit, lubricity may be insufficient. In contrast, when the kinematic viscosity at 40° C. is more than the above upper limit, required low temperature viscosity and sufficient fuel-efficient performance may not be obtained.

The viscosity index of the lubricating oil composition according to the present embodiment is preferably 200 to 270, more preferably 220 to 265, even more preferably 230 to 260, and particularly preferably 240 to 258. When the viscosity index of the lubricating oil composition according to the present embodiment is less than the above lower limit, low temperature viscosity and sufficient fuel-efficient performance may not be obtained. In contrast, when the viscosity index is more than the above upper limit, evaporability may decrease. Further, the insufficient solubility of the additive and insufficient compatibility with the sealing material may cause defects.

The MRV viscosity at −40° C. of the lubricating oil composition according to the present embodiment is preferably 10,000 to 60,000 mPa·s, more preferably 10,000 to 50,000 mPa·s, even more preferably 15,000 to 40,000 mPa·s, and particularly preferably 15,000 to 35,000 mPa·s. The MRV viscosity as mentioned herein is the MRV viscosity specified in ASTM D4684. When the MRV viscosity at −40° C. is less than the above lower limit, lubricity may be insufficient. In contrast, when the MRV viscosity at −40° C. is more than the above upper limit, required low temperature viscosity and sufficient fuel-efficient performance may not be obtained.

The lubricating oil composition according to the present embodiment can be suitably used for various lubricating oil applications because it can satisfy both high viscosity index and low temperature viscosity characteristics at high levels. Specific applications of the lubricating oil composition include lubricating oils used for internal combustion engines (internal combustion engine oils), such as automobile gasoline engines, motorcycle gasoline engines, diesel engines, gas engines, gas heat pump engines, marine engines, and power-generation engines; lubricating oils used for drive transmissions, such as automatic transmissions, manual transmissions, continuously variable transmissions, and final reduction gears (drive transmission oils); hydraulic oils used for hydraulic power units, such as dampers and construction machines; compressor oils, turbine oils, gear oils, refrigerant oils, metal-working oils, and the like.

EXAMPLES

The present invention is described in further detail below based on Examples and Comparative Examples; however, the present invention is not limited to the following Examples.

<Pour-Point Depressant>

Polymethacrylates A to K shown in Table 1 were prepared. Polymethacrylates A to K have structural units represented by formula (1), and have a methyl group as R¹ and an alkyl group having a specific number of carbon atoms shown in Table 1 as R² at a specific ratio. Table 1 also shows the weight-average molecular weight of polymethacrylates A to K, and the average number of carbon atoms in the alkyl groups having 12 or greater carbon atoms among R²s. In Table 1, an alkyl group having n carbon atom(s) is noted as Cn. For example, C1 means a methyl group.

TABLE 1 Average number of R¹ R² (mol %) carbon atoms (mol %) C20 or in C12 or Polymethacrylate Mw C1 C1 C12 C13 C14 C15 C16 C18 greater greater A 43,200 100 0 14 21 20 14 9 22 0 14.7 B 55,800 100 9 26 20 20 13 5 7 0 13.4 C 51,300 100 0 20 20 20 11 19 10 0 14.3 D 15,700 100 0 16 22 14 12 10 26 0 15.9 E 169,000 100 0 14 22 22 9 9 15 9 14.7 F 39,600 100 27 10 0 10 0 10 7 36 12.5 G 43,000 100 0 17 22 10 14 14 23 0 16.3 H 68,000 100 14 14 16 18 13 10 15 14 13.5 I 230,000 100 5 16 15 18 13 14 19 5 14.5 J 250,000 100 31 5 12 15 14 13 10 0 14.2 K 218,000 100 7 0 5 8 15 30 35 0 16.3

Examples 1 to 10 and Comparative Examples 1 to 15

In Examples 1 to 10 and Comparative Examples 1 to 15, lubricating oil compositions having the formulations shown in Tables 3 to 7 were prepared using polymethacrylates A to K shown in Table 1, and the following lubricating base oils and additive package. Tables 3 to 7 also show the kinematic viscosity at 100° C. and viscosity index of the lubricating oil compositions.

<Lubricating Base Oil>

Base oils 1 to 3 (all of which were mineral oil-based base oils) shown in Table 2 were used.

TABLE 2 Base Base Base oil 1 oil 2 oil 3 General property Kinematic viscosity (40° C.), mm²/s 15.2 15.8 15.9 Kinematic viscosity (100° C.), mm²/s 3.8 3.85 3.85 Viscosity index 150 152 140 Pour point, ° C. −12.5 −10 −20 CCS viscosity (−30° C.), mPa · s 830 850 810 (−35° C.), mPa · s 1450 1530 1420 Acid value, mgKOH/g 0.01 0.01 0.01 Evaporation amount NOACK (250° C., 14.1 13.5 1.45 1 h), mass % Sulfur content, mass ppm <1 <1 <1 Minute amount of nitrogen, mass ppm Less Less Less than 10 than 10 than 10 Urea adduct value, mass % 5.8 6.6 1.4

<Additive Package>

Additive package A comprising an anti-wear agent, a metal deactivator, an ashless dispersant, a metal-based detergent, an anti-rust agent, and an antioxidant was used.

[Measurement Method for MRV Viscosity]

The MRV viscosity of the lubricating base oils, which corresponded to SAE-10, obtained in Examples 1 to 10 and Comparative Examples 1 to 15 was measured by the methods described in JIS K 2010 “Automobile engine oil viscosity classification” and ASTM D4684 “Standard Test Method for Determination of Yield Stress and Apparent Viscosity of Engine Oils at Low temperature.” Tables 3 to 7 show the measured MRV viscosity and the presence of yield stress (Y.S.). Viscosity comparison is impossible when Y.S. is detected; thus, numbers are not noted. Detected Y.S. means a deviation from the 0W-20 standard.

TABLE 3 Example: Base oil 1 Example 1 2 3 4 5 Type of pour point depressant A B C D E Formulation (mass %) Base oil 89.5 89.5 89.5 89.5 89.5 Package A 10.0 10.0 10.0 10.0 10.0 Pour point depressant 0.5 0.5 0.5 0.5 0.5 Property Kinematic viscosity, mm²/s, (40° C.) 33.4 33.5 33.3 33.7 33.8 Viscosity index 252 253 253 253 254 Presence of Y.S. None None None None None MRV viscosity, mPa · s, (−40° C.) 21,700 23,500 20,700 22,800 17,400

TABLE 4 Example: Base oil 2 Example 6 7 5 9 10 Type of pour point depressant A B C D E Formulation (mass %) Base oil 89.5 89.5 89.5 89.5 89.5 Package A 10.0 10.0 10.0 10.0 10.0 Pour point depressant 0.5 0.5 0.5 0.5 0.5 Property Kinematic viscosity, mm²/s, (40° C.) 33.5 33.7 33.6 33.9 34.1 Viscosity index 253 255 256 256 257 Presence of Y.S. None None None None None MRV viscosity, mPa · s, (−40° C.) 22,300 24,600 21,300 23,500 18,900

TABLE 5 Comparative Example: Base oil 1 Comparative Example 1 2 3 4 5 6 7 Type of pour point F G H I J K — depressant Formulation (mass %) Base oil 89.5 89.5 89.5 89.5 89.5 89.5 90.0 Package A 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Pour point 0.5 0.5 0.5 0.5 0.5 0.5 — depressant Property Kinematic 33.3 34.6 34.2 35.3 35.5 35.4 33.1 viscosity, mm²/s, (40° C.) Viscosity index 252 256 254 257 259 258 252 Presence of Y.S. Detected Detected Detected Detected Detected Detected Detected MRV viscosity, — — — — — — — mPa · s, (−40° C.)

TABLE 6 Comparative Example: Base oil 2 Comparative Example 8 9 10 11 12 13 14 Type of pour point F G H I J K — depressant Formulation (mass %) Base oil 89.5 89.5 89.5 89.5 89.5 89.5 90.0 Package A 10.0 10.0 10.0 10.0 10.0 10.0 10.0 Pour point 0.5 0.5 0.5 0.5 0.5 0.5 — depressant Property Kinematic 33.5 34.9 34.6 35.8 36.0 35.9 33.3 viscosity, mm²/s, (40° C.) Viscosity index 255 258 257 261 263 262 253 Presence of Y.S. Detected Detected Detected Detected Detected Detected Detected MRV viscosity, — — — — — — — mPa · s, (−40° C.)

TABLE 7 Comparative Example: Base oil 3 Comparative Example 15 Type of pour-point depressant — Formulation (mass %) Base oil 89.5 Package A 10.0 Pour-point depressant 0.5 Property Kinematic viscosity, mm²/s, (40° C.) 33.5 Viscosity index 255 Presence of Y.S. MRV viscosity, mPa · s, (−40° C.) 12,500 

1. A lubricating oil composition comprising: a lubricating base oil having a kinematic viscosity at 100° C. of 3.5 to 4.5 mm²/s, a viscosity index of 145 or greater, and an urea adduct value of 2 to 7% by mass; and a poly(meth)acrylate pour-point depressant containing structural units represented by the following formula (1):

wherein in the formula (1), R¹ represents a hydrogen atom or a methyl group, and R² represents an alkyl group; based on the total amount of the structural units represented by the formula (1) contained in the poly(meth)acrylate pour-point depressant, a ratio of the structural units represented by the formula (1) wherein the R² is a methyl group is 0 to 10 mol %, and a ratio of the structural units represented by the formula (1) wherein the R² is an alkyl group having 12 or greater carbon atoms is 90 to 100 mol %; the alkyl group having 12 or greater carbon atoms in the poly(meth)acrylate pour-point depressant has an average number of carbon atoms of 13 to 16; and the poly(meth)acrylate pour-point depressant has a weight-average molecular weight of 10,000 to 200,000.
 2. The lubricating oil composition according to claim 1, wherein the ratio of structural units represented by the formula (1) wherein the R² is an alkyl group having 20 or greater carbon atoms is 0 to 10 mol %, based on the total amount of structural units represented by the formula (1) contained in the poly(meth)acrylate pour-point depressant.
 3. The lubricating oil composition according to claim 1, wherein a content ratio of the poly(meth)acrylate pour-point depressant is 0.01 to 2.0% by mass based on the total amount of the lubricating oil composition.
 4. The lubricating oil composition according to claim 1, wherein R¹ represents a methyl group.
 5. The lubricating oil composition according to claim 1, wherein the composition has a MRV viscosity at −40° C. of 10,000 to 60,000 mPa·s. 